The present application relates to a holographic recording medium and a holographic writing system and holographic reading system that use the holographic recording medium.
A holographic recording scheme records by causing the interference between the signal light including a two-dimensional data pattern and reference light and changing the physical properties of a recording layer containing a recording material in accordance with the distribution of strength of the interference fringes.
The photopolymer functioning as the recording material is characterized in that (1) the refractive index can be highly modulated, which can provide a high diffraction efficiency; (2) easy processing is allowed; (3) low noise; and (4) low costs. Thus, the photopolymer is an extremely potential material for putting the holographic recording to use.
However, photopolymer shrinks and swells largely due to a temperature change, which has an important effect on data reading when the data reading is performed at a different temperature from that of recording since the thermal shrinking/swelling of the recording layer changes the space between the interference fringes recorded within the recording layer. The refractive index of a recording material also depends on the temperature, and the change in refractive index changes the diffraction condition, which has an important effect on data reading as well as shrinking/swelling.
The degree of the effects may depend on the difference in temperature between recording and reading, the linear expansion coefficient of a recording material, the temperature characteristic of the refractive index, the numerical aperture of a lens, the size of a recording data pixel and the thickness of a recording layer.
By the way, in holographic data recording, the amount of photopolymer contributing to recording may be increased in order to increase the recording density, and the increase can be achieved by increasing the thickness of the recording layer. However, the increase in thickness of the recording layer may increase the severity of the condition (or selectivity) for the diffraction, and the effect on data reading due to a thermal deformation is larger in a thicker recording layer even with a same linear expansion coefficient.
Alternatively, the recording density can be increased by decreasing the size of recording data pixels for increasing the data capacity of one page or increasing the numerical aperture of a lens for decreasing the size of the unit hologram area within a recording material. However, like the case with the recording layer with an increased thickness, the diffraction condition becomes severer due to the shrinking or swelling of a recording material.
A method has been proposed which relates to the swelling/shrinking of a recording layer due to the surrounding temperature and compensates the effect by adjusting the laser wavelength in reading (see JP-A-2002-32001 (Patent Document 1), for example).
However, the adjustment of the laser wavelength in reading is difficult when the characteristics of a recording material such as the linear expansion coefficient and the temperature characteristic of the refractive index and the environment (including the temperature and wavelength) in writing are not available.